The accumulation of excessive heat on the rotor of an induction motor can cause severe thermal stress to the rotor structure, resulting in rotor conductor burnout or even total motor failure. Consequently, an accurate and reliable real-time tracking of the rotor temperature is needed to provide an adequate warning of imminent rotor over-heating. Furthermore, many line-connected induction motors are characterized by totally enclosed frames, as well as, small air gaps to increase their efficiencies. Therefore, the stator and rotor temperatures are highly correlated due to such motor designs, and the rotor temperature can often be used as an indicator of the stator winding temperature. For these motors, the continuous monitoring of the rotor temperature also provides a way to properly protect the motors against stator winding insulation deterioration due to over-heating. Thus, rotor temperature estimation is considered an integral part of induction motor condition monitoring, diagnosis, and protection.
There are typically four major approaches to obtaining the rotor temperature in an induction motor. The first approach makes direct measurements of rotor temperatures by way of temperature sensors, such as thermocouples, temperature-sensitive stick-ons, temperature-sensitive paint, and/or infrared cameras. These methods of measurement typically involve expensive instruments and dedicated wiring from the motors back to a motor control center. Therefore, this approach adds operating cost to line-connected induction motors.
The second approach utilizes a thermal model-based temperature estimator to calculate the rotor temperature. A lumped-parameter thermal circuit, typically with a single thermal capacitor and a single thermal resistor, is often used to emulate the thermal characteristic of a motor. For a class of motors with the same rating, the thermal capacitance and resistance are often pre-determined by plant operators or electrical installation engineers based on a set of known parameters, such as full load current, service factor, and trip class. Consequently, such thermal models cannot respond to changes in the motors' cooling capability, such as a broken cooling fan or a clogged motor casing. Therefore, this approach is incapable of giving an accurate estimate of the rotor temperature tailored to a specific motor's cooling capability.
The third approach typically applies to inverter-fed induction motors. By injecting high frequency signals into a motor, a magnetic field is created inside the motor. The electrical response waveforms are recorded, and the value of the rotor resistance or the rotor time constant is extracted. The rotor temperature information is then derived from the extracted rotor resistance and/or rotor time constant value. However, this approach requires an injection circuit and is usually impractical for line-connected induction motors.
The fourth approach derives the rotor temperature from the rotor resistance and/or the rotor time constant, based on the induction motor equivalent circuit. This technique requires the knowledge of rotor speed. This speed information is usually obtained from a mechanical speed sensor attached to the shaft of a line-connected motor. Because of the cost and fragile nature of such a speed sensor, and because of the difficulty of installing the sensor in many motor applications, speed-sensorless schemes based on induction motor magnetic saliency have been preferred. By sampling a single-phase current at steady-state motor operation, the rotor speed information is extracted from spectral estimation techniques such as fast Fourier transform. This speed information is then used in conjunction with the induction motor equivalent circuit to produce an estimate of the rotor resistance and subsequently the rotor temperature. However, accurate rotor temperature cannot be reliably obtained during dynamic motor operations, where motors are connected to time-varying loads such as reciprocating compressors or pumps. In addition, because most spectral estimation techniques require that the whole sequence of current samples, acquired at steady-state motor operation, be available for batch processing, such techniques can cause significant delay between the data acquisition and the final output of the temperature estimates.
What is needed is a cost-effective induction motor condition monitoring, diagnosis, and protection system with a highly accurate and reliable rotor temperature estimator that relies on measurements of voltage and/or current. What is also needed is an induction motor condition monitoring, diagnosis, and protection system with the ability to continuously track the rotor temperature during steady-state and/or dynamic motor operations. It is further needed to interleave the data acquisition process with the rotor temperature estimation process such that: (1) a set of voltage and current samples is acquired, (2) the corresponding rotor temperature is estimated, and (3) the estimation is completed before the next set of voltage and current samples is available.